This invention relates generally to rotation sensors and particularly to ring laser gyroscope rotation sensors. Still more particularly, this invention relates to apparatus and methods for stabilizing the frequency of light produced in a ring laser.
A ring laser gyroscope employs the Sagnac effect to detect rotation. Two counterpropagating light beams in a closed loop will have transit times that differ in direct proportion to the rotation rate of the loop about an axis perpendicular to the plane of the loop. There are in general two basic techniques for utilizing the Sagnac effect to detect rotations. A first technique is the interferometric approach, which involves measuring the differential phase shift between two counterpropagating beams injected from an external source, typically a laser, into a Sagnac ring. The ring may be defined by mirrors that direct the light beams around the path or by a coil of optical fiber. Beams exiting the path interfere and create a pattern of light and dark lines that is usually called a fringe pattern. Absolute changes in the fringe pattern are indicative of rotation of the ring. The primary difficulty with such devices is that the changes are very small for rotation rates of interest in guidance applications.
The ring laser gyroscope uses the resonant properties of a closed cavity to convert the Sagnac phase difference between the counter propagating beams into a frequency difference. The high optical frequencies of about 10.sup.15 Hz for light used in ring laser gyroscopes cause the minute phase changes to become beat frequencies that are readily measured. The cavity length must be precisely controlled to provide stability in the lasing frequency. A stable frequency is required to provide the desired accuracy in measuring rotations.
A ring laser gyroscope has a sensor axis that passes through the closed paths traversed by the counterporpagating beams. When the ring laser gyroscope is not rotating about its sensor axis, the optical paths for the two counterpropagating beams have identical lengths so that the two beams have identical frequencies. Rotation of the ring laser gyroscope about its sensor axis causes the effective path length for light traveling in the direction of rotation to increase while the effective path length for the wave traveling in the direction opposite to the rotation decreases.
Ring laser gyroscopes may be classified as passive or active, depending upon whether the lasing, or gain medium is external or internal to the cavity. In the active ring laser gyroscope the cavity defined by the closed optical path becomes an oscillator, and output beams from the two directions beat together to give a beat frequency that is a measure of the rotation rate. The oscillator approach means that the frequency filtering properties of the cavity resonator are narrowed by many orders of magnitude below the passive cavity and give very precise rotation sensing potential. To date the major ring laser gyroscope rotation sensor effort has been put into the active ring laser. Presently all commercially available optical rotation sensors are active ring laser gyroscopes.
U.S. Pat. No. 4,449,824 issued May 22, 1984 to Matthews is directed toward producing output signals representing the frequency differences between counter-circulating wave pairs circulating as two beams within the gyroscope cavity. A partially transmitting dielectric mirror forms both one of the cavity reflectors and means for extracting a small portion of each beam to be processed by cavity length control apparatus.
U.S. Pat. No. 4,482,249 issued Nov. 13, 1984 to Smith, Jr. et al. is directed toward the use of an electromagnetic wire ring resonator wherein field distributions of the electromagnetic waves are spatially rotated about the direction of propagation of such waves in said resonator. This arrangement enables waves of opposite polarization senses to resonate with different frequencies.
U.S. Pat. No. 4,229,106 issued Oct. 21, 1980 to Dorschner et al. is directed toward the use of an electromagnetic wave resonator including means to spatially rotate the electromagnetic field distribution of waves resonant thereon about the direction of propagation of such waves to enable the waves to resonate with opposite senses of circular polarization.
U.S. Pat. No. 4,141,651 issued Feb. 27, 1979 to Smith et al. discloses a four frequency laser gyroscope system for producing outputs signals representing the frequency differences between counter-circulating wave pairs circulating as two beams within the gyroscope cavity. A partially transmitting dielectric mirror forms both one of the cavity reflectors and the means for extracting a small portion of the beam. Each resultant beam is then polarization discriminated to extract the desired signal content.
U.S. Pat. No. 4,108,553 issued Aug. 22, 1978 to Zampiello et al. discloses a four frequency laser gyroscope system having parallel processing of pathlength control and detection signals. Two output signal beams each containing component of the two of the four waves circulating in the laser cavity are shown upon separate detector diodes. The low frequency component is processed to control the optical pathlength, and the high frequency component is processed to produce a signal representing the amount of cavity rotation.
U.S. Pat. No. 4,320,974 issued Mar. 23, 1982 to Ljung discloses an electronic circuit using the outputs of gyro fringe detectors as both photo detectors and preamplifiers. Thus, the same detectors are used to serve two purposes. Pathlength control simplification and cost reduction is thus realized since it is unnecessary to mount and align separate photo detectors to a gyro.
A. D. White, "Frequency Stabilization of Gas Lasers", IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. QE-1, No. 8, November, 1965 surveys the state of the art in the field of frequency stabilization of gas lasers. A brief discussion of the methods employed to determined the freuqency stability of lasers is followed by a listing of the principle causes of frequency instability. The close relationship existing between the control system design in the laser environment is pointed out. Stabilization techniques based on the use of atomic resonance and on the use of interferometers are discussed in detail.
U.S. Pat. No. 4,383,763 issued May 17, 1983 to Hutchings et al. discloses a mirror which may be internal to a ring laser gyroscope constrained to translation and whose position is controlled by a piezoelectric ceramic actuator operating as a bimorph.
U.S. Pat. No. 4,585,346 issued Apr. 29, 1986 to Ljung discloses a path length controller for a three axis ring laser gyroscope assembly that includes a power detector for each of three beams and electronic circuitry and actuators for controlling the positions of three movable mirrors to maintain the power level in each beam at a maximum value.